Answer this question truthfully
### Question: What causes maple syrup urine disease?
### Response: Maple syrup urine disease (MSUD) is caused by a gene defect. Persons with this condition cannot break down the branched-chain amino acids leucine, isoleucine, and valine. This leads to a buildup of these chemicals in the blood.
In the most severe form, MSUD can damage the brain during times of physical stress (such as infection, fever, or not eating for a long time).
MSUD also occurs in an intermittent form and a mild form. Even in the mildest form, repeated periods of physical stress can cause mental retardation and high levels of leucine.